The invention relates to a fuel injector for use in the delivery of fuel to a combustion space of an internal combustion engine. In particular, but not exclusively, the invention relates to a fuel injector of the type intended for use in a fuel system of the accumulator or common rail type, the injector being of the type controlled using a piezoelectric actuator arrangement.
In a known piezoelectrically actuated fuel injector, a piezoelectric actuator is operable to control the position occupied by a control piston, the piston being moveable to control the fuel pressure within a control chamber defined by a surface associated with the valve needle of the injector and a surface of the control piston. The piezoelectric actuator includes a stack of piezoelectric elements, the energisation level, and hence the axial length, of the stack being controlled by applying a voltage across the stack. Upon de-energisation of the piezoelectric stack, the axial length of the stack is reduced and the control piston is moved in a direction which causes the volume of the control chamber to be increased, thereby causing fuel pressure within the control chamber to be reduced. The force applied to the valve needle due to fuel pressure in the control chamber is therefore reduced, causing the valve needle to lift away from a valve needle seating so as to permit fuel delivery into the associated engine cylinder.
In order to cause initial movement of the valve needle away from its seating, a relatively large retracting force must be applied to the valve needle. In known piezoelectrically actuated fuel injectors, the large retracting force applied to the valve needle is maintained throughout opening movement of the valve needle to its full lift position. However, once valve needle movement has been initiated, a reduced force is sufficient to cause continued movement of the valve needle towards its full lift position. Known fuel injectors of this type are therefore relatively inefficient as a significant amount of energy is wasted in applying a large retracting force to the valve needle throughout its full range of movement.
It is an object of the invention to provide a fuel injector which alleviates this problem.
According to the present invention, a fuel injector comprises a valve member which is engageable with a valve seating to control fuel delivery from the injector, an actuator arrangement and a hydraulic amplifier arrangement for transmitting movement of the actuator arrangement to the valve member, the hydraulic amplifier arrangement comprising a piston member and a control chamber for fluid, whereby the actuator arrangement is cooperable with the piston member so as to apply a retracting force to the piston member, the amplifier arrangement being arranged such that, upon application of the initial retracting force, the valve member is caused to move with the piston member away from the valve seating, movement of the valve member being decoupled from the piston member following initial movement of the valve member away from the seating such that further movement of the valve member is transmitted from the actuator arrangement to the valve member through fluid within the control chamber, the amplifier arrangement thereby providing a variable amplification of movement of the actuator arrangement to the valve member.
Preferably, the amplifier arrangement comprises mechanical coupling means for coupling movement of the piston member to the valve member upon application of an initial retracting force to the piston member.
Preferably, the amplifier arrangement is arranged such that amplification of movement during further movement of the valve member away from the valve seating is determined by the relative diameters of the piston member and the valve member.
Preferably, the actuator arrangement includes a piezoelectric element or a stack of piezoelectric elements, the piezoelectric element being cooperable with the piston member so as to apply the retracting force to the piston member upon the axial length of the piezoelectric element being reduced.
During the initial stage of operation in which the piston member is mechanically coupled to the valve member the injector has an initial, relatively low amplification of movement. During a secondary stage of operation in which the piston member is mechanically decoupled from the valve member the injector has a second, relatively high amplification of movement, the second amplification of movement being determined by the relative diameters of the piston member and the valve needle.
A relatively large force is required to cause initial movement of the valve member away from its seating to commence injection but, following initial lift, a reduced force is required to cause continued movement of the valve member towards its full lift position. The present invention provides the advantage that movement of the piezoelectric element which is transmitted to the valve member is amplified by a variable amount through the full range of movement of the valve member. Thus, relatively low amplification of movement can be applied to the valve member during initial lifting of the valve member away from a valve member seating, following which a relatively high amplification of movement is applied to the valve member to continue movement of the valve member away from the seating. The invention therefore permits the different amplification requirements during the initial stage of valve member movement and during continued movement of the valve member to be achieved in an efficient manner. As the fuel injector of the present invention can be operated more efficiently, fuel consumption is improved. The invention also enables the control of valve member movement to be improved.
Preferably, the control chamber is defined, in part, by a piston bore provided in the piston member.
The injector may comprise a further chamber, whereby upon opening movement of the valve member fuel flows from the control chamber to the further chamber at a relatively low rate.
In a preferred embodiment, the injector further comprises means for substantially preventing closing movement of the valve member from being damped. In a first embodiment, said means include valve means operable between a closed position, in which a substantially fluid tight seal is provided between the control chamber and the further chamber, and an open position in which a flow path for fuel provides communication between the control chamber and the further chamber.
The valve means may include an annular valve member, and the flow path for fuel may be defined, in part, between the piston member and the valve member.
The annular valve member may be arranged to be engageable with a further seating defined by a surface of the valve member to control opening and closing of the flow path for fuel.
In a second embodiment, the means for substantially preventing closing movement of the valve member from being damped may include valve means, preferably including an annular valve member, operable between a seated position in which a restricted flow path is defined between the control chamber and the further chamber and an unseated position in which a relatively unrestricted flow path for fuel is defined between the control chamber and the further chamber.
In this embodiment, the annular valve member may be arranged to define, in part, a restricted flow path which serves to restrict the rate of flow of fuel from the control chamber during opening movement of the valve member, thereby to cause opening movement of the valve member to be damped.
Preferably, the annular valve member may have an outer surface provided with a screw thread formation which defines, in part, the restricted flow path. Alternatively, the annular valve member may have an outer surface provided with flats, slots and/or grooves to define the restricted flow path.
In another alternative embodiment, the injector may comprise damping means for damping opening movement of the valve member.
The damping means may include a restricted passage provided in the valve member, one end of which communicates with the control chamber and the other end of which communicates with the further chamber, whereby upon opening movement of the valve member fuel flows from the control chamber to the further chamber at a relatively low rate.
The injector preferably comprises a nozzle body provided with a nozzle body bore within which the valve member is movable, the nozzle body being provided with a projection which is received, in part, within a sleeve member within which the piston member slides. The piston member may be arranged to form a substantially fluid tight seal within the sleeve member.
The valve member may be shaped to include a region of enlarged diameter, the piston member being shaped to defined a further surface which is engageable with the enlarged region of the valve member so as to couple movement of the piston member and the valve member upon application of the initial retracting force, movement of the piston member and the valve member being decoupled following initial movement of the valve member away from its seating.
Alternatively, the mechanical coupling means take the form of a substantially C-shaped spring received partially within a first groove provided on the surface of the valve member and partially within a corresponding second groove provided on the piston member such that, upon application of the initial retracting force to the piston member, the spring serves to couple movement of the piston member to the valve member.
The spring is preferably arranged such that, following initial movement of the valve member away from the valve seating, the spring is able to ride within the corresponding second groove provided on the piston member, thereby permitting relative movement between the piston member and the valve member.
The stack of piezoelectric elements may preferably have an end member associated therewith, the end member cooperating with the piston member so as to transmit movement to the piston member upon the axial length of the piezoelectric element being varied.
The piston member preferably has, associated therewith, resilient bias means which serve to urge the piston member and the valve member towards a position in which the valve member is seated.
Conveniently, the resilient bias means may take the form of a spring or pair of springs arranged within a blind bore provided in the end member.
A further spring member may be arranged within the control chamber to urge the valve member towards its seating.
The end member and the piston member may be provided with means for preventing relative angular movement therebetween.
Alternatively, the end member may be provided with a spherical joint to ensure the piston member is substantially axially aligned with the nozzle body of the injector.
In one embodiment, the control chamber may also be defined, in part, by a drilling provided in the valve member. The drilling may be arranged to communicate with the restricted passage in the valve member to permit opening movement of the valve member to be damped.